docs: record comment-reduction pass in COMPLETED, add comment conventions to CLAUDE.md

docs/COMPLETED.md

@@ -3,6 +3,18 @@
 Completed milestone entries removed from `PLAN.md`. Each entry preserves its
 original Goal, Verify, and checklist points with boxes marked done.
 
-This file holds the current (1.x) cycle's landed milestones only. Nothing has
-landed in the 1.x cycle yet. For all pre-1.0 (version-0) history, see
-`docs/ARCHIVE.md`.
+This file holds the current (1.x) cycle's landed milestones only. For all
+pre-1.0 (version-0) history, see `docs/ARCHIVE.md`.
+
+### Comment-reduction pass (tree-wide, twelve parallel tracks)
+
+Cut source comment volume tree-wide: 209 files changed, net **−6,493** lines.
+Comment-only lines went from 15,073 to ~8,671, a **~42% cut** — before the
+pass, 37% of all source lines were comment-only. **Zero code drift**, verified
+across all 209 files by comparing comment-stripped hashes; the sole
+intentional exception (Daniel-approved) is two user-facing error strings in
+`src/shell/capture/capture.cpp` that lost internal milestone IDs (`M8`, `M3`,
+`M7+`). Build clean, 61/61 ctest pass. Code review surfaced 2 Major + 8 Minor
+findings — all content cut that should have survived — and all ten were
+remediated and re-gated before merge. Driver: Daniel's instruction — *"Brief
+concise engineering comments. A little why, and maybe context, never WHAT."*